But a stubborn handful of receptors has resisted every attempt at de-orphanization. TAS2R19, TAS2R42, TAS2R45, and TAS2R60 remain orphans, receptors with no known activating compounds, and their biological roles have therefore remained shrouded in uncertainty. At the same time, even among receptors classified as functional, genetic variation has produced variants labeled nonfunctional, sometimes on the basis of just one or a few amino acid substitutions. Two famous examples are TAS2R9 and TAS2R38. In TAS2R9, an alanine at position 187 yields a functional receptor, while a valine at the same position renders it unresponsive to known agonists such as the antibiotic ofloxacin. TAS2R38 is even more celebrated: its PAV variant makes people exquisitely sensitive to the bitter compounds phenylthiocarbamide and propylthiouracil, whereas the AVI variant is classed as nonfunctional and defines the classic taster-non-taster divide in human populations. Whether such variants are truly dead or have simply shifted their ligand preferences to unknown chemicals has been impossible to determine from sequence alone.

A new study by Praveen Kumar and Maik Behrens, published in Current Research in Food Science, tackles this puzzle from an unexpected angle. Instead of hunting for missing agonists, the researchers asked a more fundamental question: do these orphan and supposedly nonfunctional receptors even reach the cell surface, the location where a receptor must reside to detect compounds outside the cell? A receptor that fails to fold properly, or that gets stuck in the endoplasmic reticulum during biosynthesis, can never respond to any ligand, no matter how thorough the screening. Conversely, a receptor that traffics efficiently to the plasma membrane is structurally intact and remains a plausible candidate for future ligand discovery. The answer to this question provides a powerful clue about functionality that does not require knowing a single agonist.

To measure cell surface localization, the team employed a clever tagging strategy developed recently in their laboratory. Each receptor was engineered to carry an 11-amino-acid HiBiT sequence, a small peptide tag appended to the amino-terminal end of an sst3 export tag derived from the rat somatostatin receptor subtype 3, which is itself crucial for routing receptors to the plasma membrane. The HiBiT tag binds with high affinity to a complementary protein fragment called LgBiT, and together the two fragments generate luminescence. Because some of the commercial detection reagents are membrane-impermeable, they can only reach HiBiT tags exposed on the outside of living cells, allowing researchers to quantify exactly how much of each receptor sits at the cell surface. A second, lytic detection reagent breaks the cells open and reveals the total receptor population, permitting calculation of the fraction that successfully completed trafficking. The entire workflow runs on a fluorometric imaging plate reader, making it suitable for high-throughput analysis.

The first test cases were the taster and non-taster variants of TAS2R9 and TAS2R38. Functional assays in HEK293T-Gα16gust44 cells confirmed the expected behavior: the TAS2R38-PAV variant responded to propylthiouracil and the TAS2R9-187A variant responded to ofloxacin, while their non-taster counterparts remained silent. Importantly, adding the HiBiT tag did not abolish function, although the tagged TAS2R38 showed a slightly reduced response, a caveat the authors note openly. When the researchers then measured cell surface abundance using the luminescence assay, both functional and nonfunctional variants proved to be present at the plasma membrane. The non-taster TAS2R38-AVI variant reached the cell surface at a reduced fraction, about 6 percent of its total pool compared with 17 percent for the PAV variant, and the TAS2R9-187V variant showed only a modest reduction relative to its functional partner. Immunocytochemical staining of living, non-permeabilized cells with an anti-HiBiT antibody independently confirmed these surface-localization patterns.

These results carry significant implications for how scientists interpret receptor polymorphisms. For TAS2R9-187V, the data suggest that no gross defect in folding or routing explains its apparent nonfunctionality. Position 187 sits in the upper part of the fifth transmembrane domain, a region that could plausibly contribute to agonist binding. The authors therefore propose that rather than being a lost receptor, this variant may simply respond to a different, perhaps smaller, agonist that has not yet been tested. The bulkier valine residue could sterically reshape the ligand-binding pocket or restrict the conformational movements of the transmembrane helices needed for receptor activation. Similarly, for TAS2R38-AVI, the demonstration of structural integrity at the cell surface supports earlier speculation that this variant may have evolved altered ligand specificity, possibly responding to unknown compounds in the pulp of the Antidesma bunius fruit, which produces a peculiar bitter taste in individuals carrying the AVI haplotype.

The four orphan receptors yielded equally revealing results. All four could be detected at the cell surface by luminescence assays, but their trafficking efficiencies differed dramatically. TAS2R19 stood out as the star performer: 83 percent of its total receptor pool reached the plasma membrane, a proportion exceeding even that of TAS2R14, the human bitter receptor with the largest number of known agonists. TAS2R60 was by far the most abundantly produced receptor subtype, and although only 9 percent of it reached the surface, the absolute quantity there surpassed all TAS2Rs except TAS2R19. In sharp contrast, TAS2R42 and TAS2R45 showed negligible surface signals in immunocytochemical staining, with cell surface fractions of just 3 and 6 percent respectively. TAS2R42 is synthesized reasonably well but trafficked poorly, suggesting retention in the endoplasmic reticulum, while TAS2R45 appears to be produced at very low levels overall, raising the possibility that it may functionally represent an expressed pseudogene.

The contrasting profiles of the four orphans allow the authors to draw nuanced conclusions about why these receptors have resisted de-orphanization. For TAS2R19 and TAS2R60, biosynthesis and cell surface routing in heterologous cells clearly do not explain the absence of known agonists. Both receptors are closely related to already de-orphaned receptors in mice: the mouse receptor Tas2r135, an ortholog of TAS2R60, has 11 identified bitter agonists, and TAS2R19 is highly homologous to TAS2R20, which has 10 known agonists. Problems with coupling to the calcium signaling cascade used in the assay system also seem unlikely, given that far more distantly related receptors, including one from cartilaginous fish, have been successfully screened in the same cellular platform. The most probable explanation for these two receptors is simply that the right bitter compounds have not yet been tested, pointing toward unexplored chemical territory.

That territory, the authors argue, is vast and seriously under-sampled. Most bitter agonist discovery has focused on synthetic compounds and molecules from flowering plants, while bitter substances from animals, mushrooms and other fungi, prokaryotic organisms, and plants belonging to older lineages of the plant kingdom have barely been investigated. Recent work identifying bitter compounds from mushrooms, such as those isolated from the bracket fungus Amaropostia stiptica, illustrates the kind of neglected sources that could harbor activators for the remaining orphans. Closing this knowledge gap, the study suggests, will require intensified bioprospecting efforts guided by the growing recognition that bitter taste receptors do far more than guard the tongue. They are expressed in the gut, airways, and other tissues, where they influence hormone secretion, glucose homeostasis, and possibly innate defense, making every de-orphanized receptor a potential target for nutrition and medicine.

The HiBiT-based localization assay itself may prove to be the study’s most lasting contribution. By providing a rapid, quantitative, high-throughput readout of receptor biosynthetic integrity that requires no knowledge of agonists, the method offers a rational triage tool for the bitter taste receptor field. Receptors that traffic well, like TAS2R19 and TAS2R60, become priority targets for expanded ligand screening, while those with severe trafficking defects, like TAS2R42 and TAS2R45, may need engineered chaperones, modified expression systems, or tissue-specific organoid models to reveal their secrets. As the authors note, genetically engineered organoids derived from taste tissue or the intestine could eventually provide more natural cellular environments for such studies, overcoming limitations of immortalized cell lines that express numerous receptors endogenously. For now, the message is clear: four orphan receptors and two supposedly dead receptor variants are very much alive at the cell surface, waiting for the right molecule to wake them.

Subject of Research: Cell surface localization of orphan and nonfunctional human bitter taste receptors

Article Title: Cell surface localization of receptors considered nonfunctional and orphan bitter taste receptors

Article References: Kumar, P., & Behrens, M. (2026). Cell surface localization of receptors considered nonfunctional and orphan bitter taste receptors. Current Research in Food Science, 13, Article 101551. https://doi.org/10.1016/j.crfs.2026.101551

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101551

Keywords: bitter taste receptors, TAS2R, orphan receptors, HiBiT tag, cell surface localization, G protein-coupled receptors, TAS2R38, TAS2R9, de-orphanization, taste perception, receptor trafficking, food science

Sweet sensation in humans begins with the activation of specialized receptor proteins located on the surface of taste bud cells. These receptors, belonging to the class C G protein–coupled receptors (GPCRs) family, consist of two subunits: TAS1R2 and TAS1R3. While the general role of these proteins in sweetness perception has been established, the precise molecular mechanics by which different sweet molecules induce receptor activation has remained obscure. Previous functional assays provided indirect clues but failed to capture the dynamic structural changes crucial to signal transduction.

To overcome this challenge, the St. Jude team applied advanced cryo-EM techniques that allow visualization of protein complexes at near-atomic resolution in multiple conformational states. This enabled researchers to observe not only the receptor’s resting and ligand-bound conformers but also an elusive intermediate referred to as the ‘loose’ state. Characterized by a partial disengagement of the so-called Venus flytrap (VFT) domains—named for their distinctive clamshell-like shape—this loose conformation appears to represent the fully activated receptor state. This discovery overturns previous assumptions derived from other class C GPCRs, where the VFT domains typically remain tightly associated upon activation.

The elucidation of this loose state revealed that when a sweetener binds TAS1R2, a flexible loop within TAS1R2 inserts itself into the interface between TAS1R2 and TAS1R3, physically prying the VFT domains apart. This structural rearrangement triggers downstream signaling cascades that ultimately generate the perception of sweetness. Notably, this mechanism diverges markedly from that of other family members, emphasizing the unique adaptations of the sweet taste receptor to its functional demands.

Furthermore, detailed comparisons between sucralose and advantame binding revealed distinct interaction patterns despite both molecules achieving receptor activation. Sucralose, a chlorinated sucrose derivative, and advantame, a highly potent aspartame analogue, engage different residues and induce subtle but significant differences in receptor dynamics. This nuanced understanding of ligand-receptor specificity provides valuable templates for the rational design of sweeteners that harness maximal receptor activation without adverse metabolic consequences.

Co-first authors Xiao Chen and Haolan Wang highlighted the importance of capturing these transient structural snapshots. “The ability to visualize how various sweet molecules modulate the receptor’s conformational ensemble allows us to decode the molecular logic behind sweetness perception,” they noted. This integrative approach combining structural biology with functional assays underscores the multidisciplinary effort required to tackle complex sensory mechanisms.

The therapeutic and commercial implications of this discovery could be profound. With global concerns about sugar consumption and metabolic health, developing low-calorie sweeteners that reliably mimic sugar’s taste without harmful effects is a key priority. By identifying structural features that stabilize the active loose state, chemists can design molecules that maximize sweetness potency, minimize off-target effects, and perhaps even modulate receptor responses in novel ways.

Importantly, this study challenges established paradigms in GPCR biology and taste research, illustrating how protein conformational plasticity underpins sensory perception. The atypical activation mechanism of the sweet receptor may also inspire analogous investigations into related receptors governing other taste modalities, broadening our molecular map of gustatory processing.

Beyond academic insights, the researchers emphasized the potential societal benefits, including public health improvements through better sweetener design. They envision a future where structural-guided sweetener development contributes to reducing sugar-related disorders such as diabetes and obesity, enhancing both quality of life and healthcare outcomes.

Funding for this innovative research was provided by the National Institutes of Health and the American Lebanese Syrian Associated Charities, reflecting strong institutional commitment to deciphering molecular biology’s medical frontiers. The collaborative effort also involved contributions from The Rockefeller University and the University of Florida, underscoring the highly interdisciplinary nature of contemporary structural biology.

Overall, this landmark study from St. Jude Children’s Research Hospital represents a major leap forward in sensory biology. By combining the power of cryo-EM with rigorous biochemical analysis, it opens new horizons for understanding and manipulating human taste at the atomic level. As researchers continue to explore the sweet receptor’s conformational universe, the prospects of creating next-generation sweeteners that are both effective and health-conscious have never been brighter.

Subject of Research: Structural and functional analysis of the human sweet taste receptor TAS1R2/TAS1R3 and its activation mechanism by artificial sweeteners.

Article Title: Structural and functional studies uncover the mechanism behind sweet taste.

News Publication Date: August 4, 2025.

Web References:

• Journal article DOI: 10.1038/s41422-025-01156-x
• St. Jude Research Lab: Chia-Hsueh Lee Lab
• St. Jude Structural Biology Department: Structural Biology

References: Published in Cell Research, August 3, 2025.

Image Credits: St. Jude Children’s Research Hospital.

Keywords: Structural biology, sweet taste receptor, TAS1R2, TAS1R3, GPCR, cryo-electron microscopy, sucralose, advantame, Venus flytrap domain, receptor activation, sweetness mechanism, artificial sweeteners.